Closed circuit heat exchange system



1965 1. o. GALLAPOO 3, 0

CLOSED CIRCUIT HEAT EXCHANGE SYSTEM Filed March 2, 1964 n 64 e3 44 MAKEUP WATER N .l 56 41 m 47 INVENTOR.

a 52 51 Isaac Orren Gallapoo Y BY 6 ATTORNEY United States Patent 3,221,710 CLOSED CIRCUIT HEAT EXCHANGESYSTEM Isaac 0. Gallapoo, Barberton, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Mar. 2, 1964,"Ser. No. 348,562 4 Claims. (Cl. 122--1) The present invention relates to heat exchange apparatus and more particularly to a pair of heat exchange elements contacted by separate gaseous streams in heat exchange relationship witha fluent heat exchange medium circulated through both of said heat exchange elements.

In many combustion processes the gaseous products contain a relatively high percentage of sulphur compounds and it isadvantageous to maintain these compounds above their dew point temperatures to avoid corrosive condensation on heater parts. Advantageously, in the present invention provision is made to preheat the heat exchange medium from a separate source during the start-up and low load operating period so as to avoid corrosion due to low air and gas temperature conditions.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

Of the drawings:

FIG. 1 is a diagrammatic elevation of a vapor generating unit showing the gas and air flow circuits incorporating the heat exchange elements of the present invention; and,

FIG. 2 is a schematic showing of the closed circuit heat exchanger used to preheat the combustion air and/ or gases in the circuit shown in FIG. 1.

The invention is illustrated in the drawings as applied in a chemical recovery system wherein the residual liquor from pulp digestion is burned to recover chemical and heat values from the liquor. .It is of course understood that the invention is not limited to a chemical recovery process, but may be used in any application where low temperature gases containing sulphur compounds are encountered.

As shown in FIG. 1 a refractory lined furnace is associated with a vapor or steam generating unit 11 so that the heating gases produced by the incineration of a magnesium base residual pulp liquor, for example, may be usefully employed in the generation of vapor, such as steam. The furnace is shown as having an inclined floor 12, and is provided with a plurality of fuel and air inlet ports 13 in its upper portion. 'The combustion of the residual liquor produces hot gases which entrain solid matter consisting primarily of magnesium oxide particles which pass with the gases through a substantially unobstructed flow path in rearwardly spaced upflow and downfiow passes 14 and 15, respectively. Thereafter the gases pass over a bank of heat exchange tubes 16 for the generation of steam. A bank of superheating tubes 17 may be positioned within the bank of tubes 16, and partially cooled gases leave the steam generating unit 11 through a duct 18 which discharges into the inletportion 20 of a tubular air heater 21.

The air heater 21 is constructed and arranged with two groups 21A and 21B of upright tubes connecting upper and lower tube sheets. The tubes of each group are connected for parallel flow of heating gases therethrough, and the two groups are arranged in side by side relation and connected by a reverse bend housing 22 for series 3,221,710 Patented Dec. 7, 1965 flow of gas. Thus the heating gases entering the inlet portion 20 pass downwardly through one group 21A of tubes into the housing 22 over economizer surfaces 31 with the gases reversed in flow direction to pass upwardly through the second group 21B of tubes into an outlet hood 23. Thereafter the gases flow through a short horizontally disposed duct 24 into the inlet hood 25 of an economizer or tubular heat exchange section 26 forming a portion of the present invention. The gases are thereafter passed through a duct 27 opening to a dust separator 28, for the removal of solids entrained in the gas, and into an induced draft fan 30 for discharge to a chemical recovery system (not shown) for the absorption of S0 for example.

In the apparatus disclosed in the drawings, the normal temperature of the combustion air delivered with the fuel to the furnace 10 is advantageously of a relatively high order, as for example 800 F. The economizer 31 of the vapor generator 11 is positioned in the housing 22 so that the relatively hot gas passing through the duct 18 will be utilized for high temperature air heating purposes first, and after the partial cooling is passed over the economizer surfaces 31 and then the second group 21B of the air heater 21. While it is desirable to recover as much heat as possible from the gases of combustion, it is also important to avoid condensation of corrosive acids on any part of the heat exchange elements. More particularly it is highly desirable to avoid dew point temperatures in the dust collector 28 and the induced draft fan 30 during low load or start-up conditions. In the present invention, the combustion gas temperatures may be raised under such low load and start-up conditions to maintain the gas above its dew point temperature.

The combustion air is discharged from a forced draft fan 33 through a duct 35 into and through a preliminary air heater 34 and thence into the upper end portion 36 of the tube groups 213 of the air heater 21. The air moves across the tubes of the group 2113 of the air heater in three consecutive reverse flow paths. From the lower portion of the group 21B, the air passes horizontally into the lower portion of and then generally upwardly in three consecutive reverse flow paths over the tube group 21A of the air heater to discharge througha duct 37 to the inlet ports 13 of the furnace 10. As shown schematically in FIG. 1, the flow of the air through the air heater is directed by a series of baflles 38 to effect three transverse passes over each tube group 213 and 21A of the 'air heater. As hereafter described in greater detail, the preliminary air heater 34 and the economizer 26 are arranged to permit preliminary heating of the combustion air before and during the start-up period of the steam generating unit. This system may also be used during low load operation when the gases of combustion leaving the furnace 10 may be directed through a bypass 39 aroundthe air heater groups 21A and 21B.

The heat exchanger of this invention is provided with a substantially closed circuit flow path between the air heater 34 and economizer 26 for a separate heat transfer medium. The heat transfer medium may conveniently consist of water. It will be understood other fluid heat transfer medium may be used, if desired. The flow path and the arrangement of equipment is shown diagrammatically in FIG. 2. As shown, the economizer 26 is for-med by a plurality of tubes 40 connecting inlet and outlet headers 41 and 42 respectively where the heating gases pass over the tubes of the economizer. The heated medium leaving the economizer from header 4-2 discharges through a pipe 43 into a drum or expansion tank 44. The air heater 34 is generally similarin construction and arrangement to the economizer 2'6 and includes rows of tubes 45 connecting inlet and outlet headers 46 and 47,

respectively. The air from the forced draft fan 33 passes transversely over the tubes 45 in indirect heat transfer relation with the heating medium passing through the tubes.

The heat transfer medium passes from the tank 44 through a line 48 connected with the inlet header 46, and in leaving the outlet header 47 flows through a pipe 50 into the suction side of a recirculating pump 51. The pump discharges the medium to flow through pipe 52 to the inlet header 41 of the heater 26.

A by-pass line 53 connects the pipe 48 with the pipe 50 where valve 54 in the line 53 and valve 55 in pipe 48 are conjointly positioned in response to temperature impulses transmitted from a thermocouple 56 positioned in pipe 52. The valve positioning serves to regulate the temperature of the heat exchange medium entering the header 41.

During start-up periods, before hot gases are available to heat the medium in the economizer 26, steam may be delivered to a submerged coil 57 positioned in the tank 44. The steam will heat the heat transfer medium in the tank and the medium may then be recirculated to warm up both the air heater 34 and the economizer 26 or if desired, the economizer 26 alone.

Steam is delivered to the coil 57 through the valved pipe 58, with the condensate from the coil discharged through a valved condensate drain 59. The rate of condensate discharge is regulated by a pressure control device 60, and the rate of steam discharge to the coil is controlled by an automatic flow valve 61 which is responsive to temperature impulses transmitted thereto from an element 62 positioned in pipe 48. The tank 44 is supplied with make up medium, such as water or other selected fluid, through a valved pipe 63. Make up fluid flow through pipe 63 may be manually or automatically regulated in accordance with the fluid level in the tank. The tank is provided with the normal vent, pressure relief and discharge connections, such as indicated at 64.

As an example of the operation of the economizer 26 and preliminary air heater 34 during start-up of the furnace 10, steam at a pressure of 142 p.s.i.g., will be delivered to the coil 57. The quantity of steam delivered will be regulated responsive to temperature of the heat transfer medium in the pipe 48 as for example, an upper limit of 250 F. Thus steam will be delivered to the coil 57 as long as the temperature of the medium in the pipe 48 is below 250 F. The steam will ordinarily be delivered to the coil 57 for an appreciable period of time before the furnace is fired, so as to heat the heat transfer medium and the heating elements before air or gas flow is started. The steam condensed in the coil 57 may be usefully employed in the hot well (not shown) of the vapor generator or may be otherwise used.

With the valve 61 controlled to close at temperatures above 250 F. the valves 54 and 55 are usually con-trolled by the adjustment of the control from the thermocouple 52 so as to actuate the control at, for example, 210 F. Thus, below the control temperature the medium will flow through pipe 53 to pump 51, while above 210 F. the medium will flow through the tubes 45 and thence to the pump.

During the initial periods of unit operation the combustion gases passing through the exchanger 26 will be heated by the heat transfer material passing therethrough, so as to eliminate condensation of corrosive materials in the separator 28 and the induced draft fan 30. As the combustion gases increase in temperature and heat the medium in exchanger 26, hot medium will also pass through the preliminary air heater 34 and with the temperature of the medium exceeding the control temperature of 250 F., for example, flow of steam to the tank 44 will be discontinued.

In the normal operation of a unit of the kind described the gas entering temperature was 355 F. and after heating the medium in the heat exchanger 42 to a leaving temperature of 265 F., the gas was discharged at 265 F. In a similar set of flow conditions, the air entering the exchanger 34 was at a temperature of F., and after cooling the medium from 265 to 210 F. the heated air was discharged at 265 F.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of operation of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. A closed circuit heat exchange system comprising a tubular air heater, means for passing air over the tubes of said tubular air heater, a separate tubular heater, means for passing combustion gases over the tubes of said separate tubular heater, means for passing a fluid heat transfer medium through the tubes of and between said air heater and separate heater, a tank positioned between said heaters in the flow path of said heat transfer medium, means for passing the fluid heat transfer medium around said air heater while completing the closed circuit flow through said separate heater and said tank, means for selectively heating the heat transfer medium including a heating coil within said tank, and means for supplying heat to said coil only when the temperature of the medium entering the tubes of said air heater is below a selected value.

2. In a heat recovery system the combination comprising, a furnace, means for burning fuel in said furnace to produce a heating gas, a vapor generator connected with said furnace to receive said heating gas and to cool said gas in generating vapor, an air heater connected with said vapor generator to receive said cooled heating gases and to further cool said gases by heating air to be used in the combustion of said fuel in said furnace, a tubular air heater positioned upstream of said air heater in an air flow sense, a tubular heat exchanger positioned in the path of the cooled heating gases leaving said air heater, pipe means including a pump for passing a fluid heat transfer medium in series through said tubular air heater and said heat exchanger and back to said tubular air heater, pipe means bypassing said tubular air heater, means in the flow path of said medium from said tubular heat exchanger to said tubular air heater for heating said medium when the temperature of said medium entering said tubular air heater is below a selected value, and means for selectively controlling the flow of said medium between said tubular air heater and said bypass pipe means in response to a selected temperature of the medium entering said tubular heat exchanger.

3. In a heat recovery system the combination comprising, a furnace, means for burning fuel in said furnace to produce a heating gas, a vapor generator connected with said furnace to receive said heating gas and to cool said gas in generating vapor, an air heater connected with said vapor generator to receive said cooled heating gases and to further cool said gases by heating air to be used in the combustion of said fuel in said furnace, said air heater including sections arranged for the series flow of heating gases therethrough, an economizer section positioned intermediate the ends of said air heater in the flow path of said heating gases, a tubular air heater positioned upstream of said air heater in an air flow sense, a tubular heat exchanger positioned in the path of the cooled heating gases leaving said air heater, pipe means including a pump for passing a fluid heat transfer medium in series through said tubular air heater and said heat exchanger and back to said tubular air heater, said last named means including an expansion tank positioned in the flow path of said medium between said tubular heat exchanger and said tubular air heater, pipe means bypassing said tubular air heater, means for heating said medium in said expansion tank when the temperature of said medium is below .a selected value, and means for selectively controlling the flow of said medium between said tubular air heater and said bypass pipe means in response to a selected temperature of the medium entering said tubular heat exchanger.

4. In a heat recovery system the combination comprising, a furnace, means for burning fuel in said furnace to produce a heating gas, a vapor generator connected with said furnace to receive said heating gas and to cool said gas in generating vapor, an air heater connected with said vapor generator to receive said cooled heating gases and to further cool said gases by heating air to be used in the combustion of said fuel in said furnace, a tubular air heater positioned upstream of said air heater in an air flow sense, a tubular heat exchanger positioned in the path of the cooled heating gases leaving said air heater, pipe means including a pump for passing a fluid heat transfer medium in a series through said tubular air heater and said heat exchanger and back to said tubular air heater, said last named means including an expansion tank positioned in the flow path of said medium between said tubular heat exchanger and said tubular air heater, pipe means bypassing said tubular air heater, means for heating said medium in said expansion tank when the temperature of said medium is below a selected value, and means for controlling the flow of said medium through said tubular air heater or said bypass pipe means in response to a selected temperature of the medium entering said tubular heat exchanger.

References Cited by the Examiner UNITED STATES PATENTS 1,294,517 2/1919 Motley 23761 1,975,519 1 0/1934 Rudorfi 122-1 2,609,797 9/ 1952 Blizard 12121 2,635,587 4/1953 Dalin et al 122-32 X 2,803,439 8/1957 Fikenscher l22-1 X 2,823,650 2/ 1958 'Hedback et a1 12i2-32 3,095,861 7/1963 Norris 1'2-27 3,106,192 10/1963 Hingst 122 7 FOREIGN PATENTS 629,298 9/ 1949 Great Britain.

OTHER REFERENCES German printed application No. 1,108,400, printed June 8, 1961.

PERCY L. PATRICK, Primary Examiner. KENNETH W. SPRAGUE, Examiner. 

1. A CLOSED CIRCUIT HEAT EXCHANGE SYSTEM COMPRISING A TUBULAR AIR HEATER, MEANS FOR PASSING AIR OVER THE TUBES OF SAID TUBULAR AIR HEATER, SEPARATE TUBULAR HEATER, MEANS FOR PASSING COMBUSTION GASES OVER THE TUBES OF SAID SEPARATE TUBULAR HEATER, MEANS FOR PASSING A FLUID HEAT TRANSFER MEDIUM THROUGH THE TUBES OF AND BETWEEN SAID AIR HEATER AND SEPARATE HEATER, A TANK POSITIONED BETWEEN SAID HEATERS IN THE FLOW PATH OF SAID HEAT TRANSFER MEDIUM, MEANS FOR PASSING THE FLUID HEAT TRANSFER MEDIUM AROUND SAID AIR HEATER WHILE COMPLETING THE CLOSED CIRCUIT FLOW THROUGH SAID SEPARATE HEATER AND SAID TANK, MEANS FOR SELECTIVELY HEATING THE HEAT TRANSFER MEDIUM INCLUDING A HEATING COIL WITHIN SAID TANK, AND MEANS FOR SUPPLYING HEAT TO SAID COIL ONLY WHEN THE TEMPERATURE OF THE MEDIUM ENTERING THE TUBES OF SAID AIR HEATER IS BELOW A SELECTED VALUE. 